It is known to transport telecommunications signals over an electricity distribution or power transmission network. Patent application WO 94/09572 A1 (Norweb) describes such a network. Delivering a telecommunications service in this manner is attractive as it overcomes one of the greatest costs in providing a new telecommunications network i.e. installing cabling to each subscriber. Existing electricity distribution cabling is used to carry the telecommunications signals.
FIG. 1 shows an example of an electricity distribution network which is adapted to carry telecommunications signals. Mains electricity enters the network from an 11 kV transmission line 105 and is transformed by substation 100 into a 415V supply which is delivered over cable 120 to subscribers S1, S2, S3. A base station BS couples telecommunications signals V.sub.B, such as voice and data signals, at injection point 110 onto distribution cable 120. The telecommunications signals propagate over the cable on radio frequency carriers to transceiver units TRX at subscriber premises S1, S2, S3. In the upstream direction, telecommunications signals are transmitted from the subscriber transceiver units towards the base station.
One of the problems with using electricity distribution cables to transport RF signals results from the structure of the cable. The inner section of the cable comprises a group of phase lines, each line carrying one of the three supply phases. At radio frequencies (RF) the capacitance which exists between these separate lines causes signals which originate on one line to leak or couple onto neighbouring lines. The process of coupling between phase lines introduces a phase shift. Therefore, after propagating along the lines, the components of a communications signal on each line will no longer be in phase with each other but will be of different phase and amplitude. This causes problems with receiving equipment.
A further problem with using power lines to transport telecommunications signals is that of interference from electrical equipment connected to the lines. Thermal noise on the lines is generally at a much lower level than other sources of interference such as broadband interference. Broadband interference occurs periodically, occupying the same frequencies as the RF carriers used to carry telecommunications signals, and occurring at levels which are high enough to corrupt the wanted signals. Interference signals propagate along the network along with the wanted RF signals, and this reduces the signal to interference ratio at the base station receiver. Some of the sources of interference are domestic appliances, particularly appliances having a motor, businesses with machinery, street lighting and other street furniture. In addition, the emissions of external sources such as radio transmitters can be picked up by the network. External sources are a particular problem where part of the distribution network uses overhead lines.
European Patent Application EP 0 063 296 (Westinghouse) describes a communication system for a three-phase power distribution network in which a remote device has three receiver front ends and a demodulator for independently reconstructing three received signals, one from each line.
U.S. Pat. No. 4,918,422 describes an apparatus for extracting inbound information generated line-to-line in a multi-phase electric distribution system. Signalling is achieved by current pulses.
JP 02-177,731 describes a receiver for a distribution line which eliminates 3-phase noise.